In the paper making industry, different kinds of roll sets are used for different purposes. A roll set comprises at least two rolls which form a nip through which a paper or a cardboard web is guided. Roll sets comprising at least one soft roll are used mainly in calenders and size presses. A size press is used for impregnating the paper web with glue or for coating the web with a pigment. A modern size press comprises two rolls coated with a soft rubber coating and means for applying coating mix to the surfaces of the rolls. The web travels through the nip formed by the rolls, and the coating is formed on both sides of the web simultaneously when the rolls covered with the coating mix are pressed against the web.
In soft calenders, a roll set comprises a soft roll covered with a polymer material and a backing roll made of steel, and the calender typically has two roll sets for calendering both sides of the web. In some applications, a roll set comprising three rolls may be used. This arrangement consists of two soft rolls and a backing roll. The backing roll forms a nip with both of the soft rolls, and the web passes successively through both nips and is thereby calendered twice. In a supercalender, a roll set comprises even more rolls, and a soft roll is always placed between two hard backing rolls. Since polymer materials presently available do not endure two or more nips on a same roll, the soft coating of the supercalender rolls is made of cotton or paper discs which are tightly pressed on the core of the roll.
One relevantly frequent problem with rotating roll sets is oscillation of the nip formed by rolls. Oscillation of a nip formed of two or more hard steel rolls can be avoided to a large extent by balancing and proper dimensioning, but when the web speeds of paper making machinery have increased, it has been noted that a nip formed by two soft rolls or by a soft roll and two or more hard rolls tends to start to vibrate at certain web speeds. The reason for this has not been known so far. These oscillations could be easily avoided by changing the web speed of the machine so that the vibration does not coincide with the natural frequency of the roll set. However, size presses and soft calenders, in which soft rolls are most commonly used, are almost without exception on-machine apparatuses. It is obvious that the paper makers want to run their machinery at a speed that results in the best quality and productivity. For example, a head box usually works best at a certain speed. This speed depends on the thickness of the web and other variables and is different for each kind of paper. For this reason it is not acceptable to change the speed of the paper machine to eliminate vibration of a size press or a calender. Speed changes also increase risk of web breaks which have a strong influence on the productivity of the machine. Changing and controlling the speed of an entire manufacturing line is very difficult, and it is preferred to run the machine on a steady speed.
The tendency of the machine to vibrate can be diminished by using rolls which have a thinner coating and a thicker core. In practice this solution is not very good since a thicker core leads to heavier rolls. The thickness of the coating can be decreased only to a certain extent since rolls have to be grinded at regular intervals due to wear and deformation of the coating. Furthermore, size presses and calenders are designed to use rolls having a certain kind of coating and use of different kinds of rolls in existing apparatuses can be difficult.